1. Field of the Invention
This invention relates to an aircraft nacelle and more particularly to a device that is provided on an aircraft nacelle to reduce, cancel or reverse thrust that is produced by a propulsion unit.
2. Description of the Related Art
An aircraft propulsion unit comprises an engine with, on the one hand, a fan that comprises a rotor that is equipped with blades and a stator that is equipped with paddles, and, on the other hand, a primary duct in which compressor stages, a combustion chamber, and turbine stages are arranged in the direction of the air flow. The engine is arranged in a nacelle that comprises an air intake upstream from the fan and a secondary duct downstream from the stator of the fan.
In general, the nacelle consists of two junctions that are arranged in the secondary duct and along angular positions that are respectively greater than 12 o'clock and less than 6 o'clock. These two junctions constitute aerodynamic fairings with structural elements and with propulsion unit systems that connect the primary duct to the rest of the nacelle. When the power plant is provided under the wing, the mast that attaches the propulsion unit to the wing is partially arranged in the upper junction to connect the primary duct to the rest of the propulsion unit.
So as to reduce the noise pollution, certain portions of the surfaces of the secondary duct comprise coatings for the acoustic surface treatment.
A propulsion unit generally comprises a device for reversing thrust that makes it possible to deflect at least a portion of the flow so as to reduce, cancel or reverse the thrust that is produced by said propulsion unit using one or more moving physical obstacles.
According to an embodiment that is described in particular in the document EP-1515035, the nacelle comprises at least one moving part that can move translationally toward the rear of the nacelle so as to prepare, between the stationary parts and said at least one moving part, at least one radial opening toward which at least a portion of the flow that flows into the secondary duct can be deflected. According to this embodiment, the secondary duct has suitable shapes, in particular bent so that the moving part comes into contact with the inside wall of the secondary duct so as to seal said duct when the moving part is moved backward in translation. Even if this approach simplifies the design of the device for reversing thrust, it imposes an additional constraint on the secondary duct, in particular a particular shape that tends to reduce the performance levels of the propulsion unit.
According to another so-called variant with a pivoting door, the nacelle comprises a moving part that is called a door that can pivot relative to its axis so as to generate a radial opening after pivoting, whereby said door comprises a part that projects into the secondary duct so as to form an obstacle that can deflect at least a portion of the flow that flows into said secondary duct in the direction of the radial opening.
According to another so-called cascade variant, the nacelle comprises at least one moving part that can move toward the rear of the nacelle in translation so as to provide—between the stationary parts and said at least one moving part—at least one radial opening, as well as flaps that are provided in the secondary duct, articulated relative to the moving part, able to occupy a first deployed position in which they at least partially stop the secondary duct so as to deflect the air flow in the direction of the radial opening and a second retracted position, flattened against the surface of the nacelle. Connecting rods are generally provided for maneuvering the flaps, whereby one of the ends of the connecting rods is connected to the flap and the other to the power plant.
The pivoting-door-type or cascade-type thrust reversers are not satisfactory for the following reasons.
These configurations limit the inside surface of the nacelle that can comprise a coating for the acoustic treatment. Actually, the connecting zones between the stationary parts and the moving parts, the zones at which the joints of the pivoting elements (doors or flaps) are provided, cannot comprise a coating for the acoustic treatment. In the case of a cascade reverser, the untreated surfaces can represent on the order of 20% of the inside surface area of the nacelle.
These configurations also produce aerodynamic losses in flows that flow into the secondary duct due to the numerous connections between the stationary parts and the moving parts and numerous obstacles that are present in the flow, such as the connecting rods for the flaps of the cascade reversers.
These configurations also produce adjustment difficulties between the stationary parts and the moving parts, in particular those that are used to deflect at least a portion of the flow because the deformation of said moving parts varies based on their position.
Finally, the presence of numerous pivoting elements, in particular in the case of a cascade reverser, causes the mass of the nacelle to increase because of the multitude of articulation and control systems and reinforcement of the structure of the nacelle so as to ensure the absorption of forces disseminated over the entire periphery of said nacelle and in particular over the moving part of the nacelle (rear frame, force expulsion, etc.).
The patent application WO 2007/003749 proposes a solution that consists in providing doors in the rear part of the nacelle. This solution allows a better acoustical treatment of the inside surfaces but leads to increasing the thickness of the nacelle in the rear part.